Methionine is a human essential amino acid, so it must be provided by its diet. But its major market is the one of animal nutrition for which it is produced in hundreds of thousands of tons per year. It is essentially manufactured by chemical synthesis.
One of the known chemical processes for obtaining methionine, see for example U.S. Pat. No. 2,557,920A, includes a step of converting methionine hydantoin by alkaline hydrolysis, or saponification, into methioninate salt, then neutralizing the latter into methionine. The hydrolysis of the methionine hydantoin is carried out in the presence of soda and/or of sodium carbonate, in aqueous phase. It occurs as a simultaneous removal of carbon dioxide and ammonia which leads to the methionine salt, sodium methioninate. Carbon dioxide and ammonia are removed from the hydrolysis medium, then the hydrolysis reaction medium containing the methioninate salt is neutralized by sulfuric acid to lead to methionine. The latter is then separated and purified by crystallization.
The conventional reaction conditions of the aforementioned steps are as follows:
The alkaline hydrolysis may be carried out in the presence of an excess of sodium hydroxide, from 1.3 to 3.5 eq., preferably from 1.7 to 2.5 eq., for temperatures comprised between 150 and 200° C.
After removal of ammonia and carbon dioxide from the saponification medium, the neutralization is generally carried out by adding concentrated sulfuric acid until a pH comprised between 3 and 6 is obtained at temperatures comprised between 70 and 130° C. Methionine can then be purified by crystallization, thereafter, by cooling. Methionine is then separated from formed sodium sulfate, by filtration at 30-50° C.
The predominant drawback of this synthesis is the formation of large volumes of sodium sulfate, at the end of the neutralization, which cannot be recycled in the process for manufacturing methionine and which are difficult to recover.
The prior art provides some answers to this problem, with processes for manufacturing aqueous solutions of sodium methioninate which are used instead of solid methionine. According to these processes, a saponification of the methionine hydantoin in the presence of soda is carried out, and a portion of sodium carbonate formed during the alkaline hydrolysis is separated and can be recycled in the saponification step, enabling to obtain a methioninate aqueous solution less concentrated in sodium carbonate. Thus, according to U.S. Pat. No. 4,391,987A, sodium carbonate is separated by cold precipitation, at a temperature ranging from −10° C. to 5° C. This has the disadvantage of using refrigerants and having to cool a saponification solution being at more than 100° C., resulting in a significantly high energy cost. U.S. Pat. No. 6,126,972A provides an improvement of this separation by concentrating the saponification solutions in order to hot-precipitate (100-130° C.) sodium carbonate and separate it by hot-filtration (100-130° C.). The purpose of this separation is to obtain sodium methioninate which can be granulated thereafter via a fluidized bed.